Continuous maize and maize/soybean rotations represent the dominant agricultural land use in the north-central USA.   Agricultural practices such as no-tillage and legume based crop rotations are prescribed best management practices that promote soil C-sequestration mitigation of global warming.   The full global warming potential (GWP) of these systems requires an accounting of the net ecosystem exchange (NEE) of carbon (C), trace gas emissions (N₂O and CH₄), fate of exported C and the embodied C-costs of fossil-fuel emissions associated with production costs.    These measurements have been made at the landscape level in three production-scale fields (each ~65 ha) to include: (i) an irrigated continuous maize system, (ii) an irrigated maize-soybean rotation, and (iii) a rainfed maize-soybean rotation.  In a second study we have been comparing intensive with conventional fertilizer and crop management practices optimized to exploit crop yield potential on GHG emissions, soil C sequestration and global warming potential.   Results show that there are large differences in primary productivity and ecosystem respiration due to differences in water supply and crop rotation and that there are also large differences in the embodied “C-costs” that contribute to the overall GWP of these systems.  The intrinsic “C-costs” from trace gas emissions and fossil fuel use comprise 25 to 30 % of annual NEE and exacerbate the GWP of these systems.  Net soil C sequestration varied between -100 to +175 g C m^-2 yr^-1 in maize years and -240 to -170 g C m^-2 yr^-1 during the soybean year.  The GWP of these systems was positive with emissions from fossil fuel use and N₂O emissions in excess of soil C sequestration.   The GWP of continuous maize was lower than that of maize-soybean systems.  The greatest potential for GHG mitigation was with maize that is converted to ethanol.